The present invention relates to an arrangement for detecting the condition of a high pressure gas discharge lamp upon switching on, in particular, the lamp temperature.
U.S. Pat. No. 4,240,009 describes a circuit for operating a gas discharge lamp which determines, adjusts and controls the power supplied to the lamp on the basis of a characteristic curve. The control unit has a power supply circuit which is controlled and regulated by a voltage-current characteristic, so that the lamp is kept in operation at a certain power, in particular, at a constant power. With the circuit, the power is determined by a wattmeter which measures the power supplied by a battery to a regulated inverter circuit on the basis of the applied voltage and the resulting current and generates control signals for the inverter circuit on the basis of the predetermined voltage-current characteristic. When a constant power is maintained for the lamp, the respective operating point is on the power hyperbola.
The Vedilis curve is given in general as the current-voltage curve in the sense of a setpoint curve for controlling a high pressure gas discharge lamp in startup and during operation. The Vedilis curve is described on page B 1/3 of xe2x80x9cSystem Specifications for Field Testxe2x80x9d of the VEDILIS Eureka Project 273, where it is given as the current-voltage characteristic for gas discharge lamps used in motor vehicles. xe2x80x9cVedilisxe2x80x9d is an abbreviation for xe2x80x9cvehicle discharge light system.xe2x80x9d According to this document, to regulate the lamp power of a gas discharge lamp, lamp voltage U, for example, is measured in the startup phase or in the operating phase, and corresponding lamp current I correlating with respective lamp voltage U is determined from the Vedilis curve. This lamp current I then serves as the setpoint for the current control circuit with which the lamp power is regulated at a constant level, e.g., 35 W.
In general, in order to use a gas discharge lamp in an automotive headlamp, the desired quantity of light must be supplied within the shortest possible amount of time after switching on the control unit. This is referred to as a rapid lamp start. As indicated by the Vedilis curve, the gas discharge lamp can be operated with a certain overload immediately after being switched on for this purpose. Then after reaching a certain voltage, this overload is reduced in accordance with the lamp voltage. This reduction follows the characteristic curve. It is clear that such an overload must not destroy the lamp or shorten its lifetime. This requirement must be met for all operating conditions.
After lighting the high pressure gas discharge lamp, it is thus operated with a defined overload in rapid lamp start, so that it delivers the required quantity of light as rapidly as possible. It is important in starting the lamp in general and in restarting it to know its condition, in particular the temperature of the lamp. Lamp start is better when the correct overload is applied, depending on the prevailing lamp temperature. The amount and duration of the overload depend on the condition of the lamp. For example, if the lamp has been turned off for a long time and is therefore cold, then it must be operated with a greater overload over a longer period of time than if it has been off only briefly and thus is still hot when restarted.
To ascertain the condition of a high pressure gas discharge lamp, the temperature can be deduced indirectly by charging and discharging a special capacitor. The capacitor is charged when the lamp is turned on, and the capacitor is discharged when the lamp is turned off. The on and off times can be determined on the basis of the voltage on the capacitor, and thus the condition of the lamp can be deduced. A disadvantage here is the additional expense in terms of hardware for the capacitor and other required components. The time constant is large and is in the range of seconds, e.g., approximately 10 S.
Alternatively, the control unit may also be left running even while the lamp is turned off, to determine the lamp temperature by determining the on and off times. This possibility requires the control unit to be supplied with power continuously and therefore it may have to be provided with a special terminal. Here again, the disadvantage is that additional suitable components and power must also be provided.
An arrangement according to the present invention has the advantage over the related art of lower component costs while achieving good results in determining the lamp condition upon turning on. The condition of the high pressure gas discharge lamp can be determined although it is not known directly how long the lamp had been turned on and subsequently turned off. Thus, the control unit need not remain running while the lamp is turned off to determine the off time.
In principle, it is provided according to the present invention that, to detect the lamp condition upon turning on, the control unit shall inject a defined current into the lamp, measure the resulting voltage drop, the lamp voltage, and analyze it, or as an alternative, the control unit may apply a defined voltage to the lamp and measure and analyze the lamp current flowing in so doing, and with both alternatives, the control unit then starts the lamp in accordance with the condition of the lamp so detected.
According to one embodiment of the present invention that is especially advantageous and expedient for both alternatives, the control unit injects the current and performs the respective voltage measurement or applies the voltage and measures the respective current and performs the respective analysis shortly after the lamp is turned on, in particular, immediately after starting the lamp. It is especially expedient that according to another embodiment of the present invention, the control unit performs the measurement operations in the load circuit of the lamp, because it thus does not cause distortion of results and does not require any additional components. In an advantageous manner, the control unit stores the measurement results for the respective lamp start.
According to an advantageous refinement of the present invention, the control unit performs the measurement and analysis operations multiple times, in particular, in rapid succession, and forms a consolidated value from the results. According to an expedient embodiment of this refinement, the control unit forms the average as the consolidated value. In an advantageous refinement, the consolidated value or the average is stored by the control unit. An especially advantageous embodiment of this refinement provides for the control unit to perform the measurement and analysis operations so that the lamp condition can be detected continuously.
In another expedient embodiment of the present invention, it is also possible for the control unit to analyze the variation of the measured quantity, in particular its slope, to ascertain the instantaneous lamp condition.
An especially expedient, advantageous and flexible embodiment of the present invention provides for a microcontroller with a memory to be provided in the control unit, so that the measurement operations can be controlled and the measurement results can be stored with its help.
The arrangement according to the present invention can be used in an especially advantageous and expedient manner with high pressure gas discharge lamps used in motor vehicles.